Almost all 35 mm theatrical motion picture film stock is constructed in accordance with ISO (International Standards Organization) and ANSI (American National Standards Institute) standards which call for film stock having two rows of perforations at a nominal pitch of 0.187 inch. The film frames are spaced apart by four perforations and the film is normally run at 18 inches per second or 90 feet per minute. Most commercial theatrical feature films for the U.S. market are projected in accordance with an Academy ratio wherein the projected image has a width of 0.825 inch and a height of 0.446 inch. This height corresponds to 2.385 perforations.
In order to save film, there have been attempts to shoot and project film where each frame at a height of two or three perforations, instead of the century-old standard of four perforations. The two perforation system was rejected on the grounds that the frame had a smaller area than previously, and therefore gave an image of lesser quality. However, the three perforation system has found some use in the television field, and theatrically in Europe. There has recently been a proposal to space the frames at 2.5 perforations.
A film transport for use in projectors, typically includes a sprocket wheel mounted on a drive shaft, the drive shaft being turned 90.degree. in intermittent steps at a rate of twenty-four times per second, by a Geneva mechanism. A sprocket wheel and drive shaft are mounted with high precision, since even small irregularities of film movement can cause noticeable movement of the greatly magnified image projected on a screen.
If films with images spaced at different numbers of perforations are to be used with the same projector or other film transport, then it is necessary for an operator to rapidly replace one sprocket wheel with another. The majority of 35 mm theatrical projectors are presently used to advance film in steps of four perforations each, by 90.degree. of rotation for each step. Typically, they use sixteen tooth sprocket wheels having lands (where the film side is supported) of a diameter of approximately 0.950 inch. To advance film in steps of three perforations each, a twelve tooth sprocket wheel is required, which has lands of a diameter of about 0.712 inch. To advance film in steps of 2.5 perforations, a ten tooth sprocket wheel is required, which has a land diameter of about 0.594 inch. In one example, two film formats are to be projected, the first requiring the film to be advanced in steps of four perforations and the other requiring the film to be advanced in steps of 2.5 perforations. It would be desirable if a changeable sprocket wheel assembly were available to enable different size sprocket wheels to be established on a drive shaft, where the changeover was simple and obvious and performable without tools by unskilled operators, and if each sprocket wheel was mounted with high precision to assure accurate film movement and avoid wear. Such operators may, for example, be paid close to a minimum wage and sell refreshments when not operating a projector.